1. Field of the Invention
This invention relates to improvements in electroluminescent lamps, particularly to an improved flexible electroluminescent lamp having a transparent electrode with improved conductivity and transparency characteristics.
2. Description of the Prior Art
Generally speaking, one type of electroluminescent lamp is made by embedding an electroluminescent phosphor in an organic resinous sheet and sandwiching the sheet between electrodes, one electrode of which is transparent to the light emitted by the phosphor. In the prior art, the transparent light transmitting electrode is generally either a layer of transparent metal oxide such as tin oxide or indium oxide or a deposit of a thin metal layer. Where a thin metal layer is employed as the transparent electrode, a compromise must be made between the light transmissivity and resistivity of the electrode. This compromise has heretofore precluded the formation of an electrode having acceptable levels of transmissivity and resistivity. Accordingly, as disclosed in U.S. Pat. No. 3,274,419 (Roth), typical prior art flexible electroluminescent lamp constructions employ transparent metal coated glass strands as the light transmitting electrode. Such constructions present an improvement over earlier employed grids of metal wires. Both such constructions have the disadvantage in that the grids or strands obscure a portion of the light transmitted from the phosphor layer. Furthermore, the glass strands are fragile, difficult to connect, and inhibit the frequencies at which the lamp may be driven. Furthermore, such constructions tend to establish a nonuniform electric field across the phosphor layer which results in a less efficient device.
Particularly with respect to flexible electroluminescent lamp constructions, considerable work has been directed to developing evaporated transparent metallic conductors. Such attempts, however, have not resulted in a viable lamp in that the conductivity and required transparency have not heretofore been obtainable in a single construction. For example, continuous electrode structures of stannic or indium oxide deposited on glass using a high temperature process have typically produced values of transparency of approximately 85% while having a resistivity on the order of about 100 ohms/square. Such electrodes may be suitable for rigid panels and flexible, fragile glass fiber paper substrates but, because of the high temperature deposition requirements, are unsuitable for use with flexible polymeric film constructions. Where evaporated electrodes such as gold have been tried in the past, if the thickness of the gold film is increased sufficiently to obtain a resistivity sufficiently less than 100 ohms/square, then the transmissivity typically decreases to approximately 50%.